1). Field of the Invention
This invention generally relates to a wafer cutting apparatus and, more specifically, to a wafer cutting apparatus which includes a saw for cutting a wafer wherein the saw is controlled by a variable of the wafer.
2). Discussion of Related Art
FIG. 1 illustrates a conventional silicon wafer 10. The manufacture of semiconductor chips involves the formation of a number of integrated circuits 12 on such a silicon wafer 10. The integrated circuits 12 are usually positioned in a matrix of rows and columns. The integrated circuits 12 are then severed from one another in a subsequent sawing operation.
FIG. 2A illustrates a portion of a wafer of the above kind having adjacent integrated circuits 12A and 12B with a saw street 16 defined between the integrated circuits 12A and 12B. A guard ring 18 is made around each integrated circuit 12A and 12B. The guard rings 18 form edges of the saw street 16.
The wafer 10 has an undulating upper surface which may be due to the manufacturing of the integrated circuits 12A and 12B or due to mounting of the wafer 10 for purposes of sawing. A V-shaped groove 20 is sawed through the saw street 16. A sawing blade (not shown) having a V-shaped edge is used for sawing the groove 20. The sawing blade is progressed through the saw street 16 without compensating for differences in height of the wafer 10 so that the width of the groove 20 varies along its length. Variations in width of the groove 20 may cause severe damage to the guard ring 18 and the integrated circuits 12.
FIG. 2B illustrates a portion of a wafer 22 with portions of integrated circuits 24 and 26, guard rings 28, and a saw street 30 as in FIG. 2A. The saw street is made of silicon and one or more objects 32 of a different material than silicon is located in the saw street 30. The objects 32 are typically made of aluminum, aluminum-copper, tungsten or copper. The objects 32 may include test features such as thickness monitors, resistance monitors, device monitors, fabrication alignment marks etc.
A sawing blade may progress through the silicon saw street 30 without causing major damage to the guard ring 28. However, when the sawing blade starts sawing through the objects 32, major damage to the guard ring 28 and the integrated circuits 12A and 12B may result. Damage to the guard ring 28 and the integrated circuits 12A and 12B is typically in the form of delamination of different layers of the wafer 10 and may be due to a number factors including the material characteristics of the objects 32, the rotational speed of the sawing blade, the speed at which the sawing blade progresses through to the saw street 30, the accumulation of debris, plate quality, past usage of the blade or variations in temperature of the wafer 10 while being cut.
A wafer cutting apparatus which includes a wafer saw, a detector, and a control unit. The wafer saw includes a wafer holder. The detector detects a variable of a wafer being held by the wafer holder. The control unit utilizes the variable to control the saw.